The present invention relates to a power factor correction circuit and more particularly, to a power factor correction circuit for improving a power factor of a switching power supply designed in bridge topologies in order to comply with the requirements of Class A or Class D as stipulated in harmonic current rules IEC-1000-3-2.
A typical switching power supply is shown in FIG. 1. The supply comprises an AC/DC rectifier 1, and a DC/DC converter 2 in which an electrolytic capacitor C3 is connected as a filter for the bridge rectifier BD1.
FIG. 5 discloses a circuit structure in which the DC/DC converter 2 shown in FIG. 1 is a half-bridge converter. In accordance with the design structure shown in FIG. 5, a bridge rectifier BD1 is used to rectify the AC power VS1. A capacitor C3 is then used to filter the rectified power and generates a DC voltage VC3. The capacitor C1 and the capacitor C2 are connected in a common node to form a voltage divider. Therefore, the voltage in the common node between the two capacitors is VC3/2.
FIG. 7 is a time chart for the PWM signals, VHG and VLG, which are driving signals for the switch Q1 and the switch Q2 shown in FIG. 5, respectively. Both PWM signals VHG and VLG are low (low voltage) for 0xe2x89xa6txe2x89xa6t1. At this time, the switch Q1 and the switch Q2 are both turned off. Therefore, the output voltage VO is supplied by the capacitor C4.
When t1xe2x89xa6txe2x89xa6t2, the PWM signal VLG is high (high voltage) and the PWM signal VHG is low (low voltage). At this time, the switch Q1 is turned off and the switch Q2 is turned on. The current flows through the capacitor C1, the primary winding P1 and the switch Q2 to the ground. Under this situation, the transformer transfers the power from the primary winding P1 to the secondary winding S1 to supply power to the capacitor C4 and output a voltage VO.
When t2xe2x89xa6txe2x89xa6t3, the switch Q1 and the switch Q2 are both turned off. At this time, the operation state of the circuit is same as the operation state of the circuit at Oxe2x89xa6txe2x89xa6t1.
When t3xe2x89xa6txe2x89xa6t4, the PWM signal VLG is low (low voltage) and the PWM signal VHG is high (high voltage). At this time, the switch Q1 is turned on and the switch Q2 is turned off. Therefore, the current flows through the switch Q1, the primary winding P1 and the capacitor C2 to the ground, The transformer transfers the power from the primary winding P1 to the secondary winding S2 to supply power to the capacitor C4 and output a voltage VO.
The power switching cycle described above is then performed repeatedly to supply power to a loading. On the other hand, the output voltage VO is transferred to a feedback system 12. The feedback system 12 feeds a signal back to the high-frequency pulse signal control circuit 50 to modify the duty cycle of the PWM signals VHG and VLG. For example, if the power supplied to the load is insufficient when the output voltage is lower than a required value, the feedback signal enlarges the duty cycle of the PWM signals VHG and VLG to increase the conduction time of the switch Q1 and the switch Q2. Therefore, the time for transferring power from the primary winding to the secondary winding of the transformer T1 is increased. In other words, the power supplied to the secondary winding is increased. The output voltage VO is therefore also increased. Finally, the output voltage VO again attains the required voltage. This means, however, that the power supplied to the load is overdriven when the output voltage is higher than the required value. In this situation, the duty cycle of the PWM signals VHG and VLG should be reduced.
Note that the input current Ipc in FIG. 5 is a pulse current as shown in the graph of FIG. 2. The power factor of the conventional switching power supply is significantly decreased (e.g., approximately 50%) due to the distorted input current, and the total harmonics distortion (hereinafter referred as THD) is even higher than 100% after the rectification performed by the AC/DC rectifier 1 shown in FIG. 1. As a result, the total harmonics is seriously distorted, the quality is poor, and, even worse, precious energy is wasted.
Thus, many countries have promulgated a number of harmonic current rules (e.g., IEC-1000-3-2) which specify the current wave shape of the power supply for manufacturers to obey in order to improve the efficiency and quality of the power source being supplied.
As such, various designs of power factor correction circuits have been proposed by researchers in order to improve power factor of the conventional switching power supply. Two examples of typical prior art are described in the following:
1. Inductor Type Power Factor Correction Circuit
As shown in FIG. 3, the prior art discloses a design in which a low frequency large winding L1 is in series between a bridge rectifier BD1 and an electrolytic capacitor C1. The winding L1 and the capacitor C1 form a low pass filter to rectify the input current of a DC/DC converter 2. Such design is similar in function to the ballast for correcting the power factor of a fluorescent lamp. However, winding L1 relatively large, has only a limited power factor improvement, and creates an abnormally high temperature during operation.
2. Active Type Power Factor Correction Circuit
As shown in FIG. 4, the prior art discloses a design in which the AC/DC rectifier is redesigned to form a two-stage circuit with the DC/DC converter 2. Further, a complex control circuit 11 and a large switch element Q1 are added therein to improve the power factor. However, it is relatively complex in circuit design and is expensive to manufacture.
Many power factor correction circuits have been developed based on the basic concepts involved in the two examples of prior art mentioned above, and with similar drawbacks.
In accordance with the foregoing description, there are many drawbacks in the conventional power factor correction circuit. For example, the circuit structure depicted in FIG. 3 is relatively large, while the circuit structure depicted in FIG. 4 is relatively complex in circuit design and is expensive to manufacture.
Therefore, the main purpose of the present invention is to provide a power factor correction circuit with a high power factor.
Another purpose of the present invention is to provide a power factor correction circuit to solve the problems existing in the prior art.
A further purpose of the present invention is to provide a switching power supply structure that is small and economical to manufacture. It is an object of the present invention to provide a power factor correction circuit comprising a series connection of a bridge rectifier and a first capacitor. The first capacitor, a winding and a first switching device are connected in series. The first switching device is the low-side switching device in a bridge converter and is connected with a first anti-parallel diode. The first switching device, a second switching device and a second capacitor are also connected in series. The second switching device is the high-side switching device in the bridge converter and is connected with a second anti-parallel 5 diode. The second capacitor acts as a boost capacitor in the PFC circuit and provides the DC operating voltage for the bridge converter. The winding can be one additional winding of the main transformer in the bridge converter or an independent inductor. Further, the power factor of the off-line switching power supply is increased to above 0.9 by appropriately selecting the value of the first capacitor and the winding in order to comply with the requirements of Class A or Class D as stipulated in harmonic current rules IEC-1000-3-2. Furthermore, the inserted PPC circuit does not affect the normal operation of the bridge converter.